Lubricating oils are critical to the operation of the machinery of the world today.
Desirable characteristics of lubricating oils include their ability to maintain thermal and hydrolytic stability, while exhibiting swelling to seals (hereinafter “seal swell”) to ensure proper functioning of the seals and to prevent loss of fluid and/or hardening of the seals as well as premature decomposition of the seals.
The use of lubricating oils in combination with various additives such as antioxidants, and wear agents, and corrosion inhibitors to provide a fluid that will meet the particular industrial oil application is known. However, in certain circumstances, the minimum performance requirements of an industrial application cannot be met by a fluid formulated from a mineral oil and commercially available additives. In such circumstances, poly-alpha-olefin (hereinafter “PAO”) and combinations of the PAOs and esters have been used as a synthetic substitute by those of skill in the art. See for example U.S. Pat. Nos. 4,992,183; 5,519,932; 5,648,108; and 5,571,445. However, fluids formulated from esters and PAOs exhibit decreased thermo-oxidative and hydrolytic stability.
More recently, oil refiners have discovered that the addition of process steps, such as severe hydrotreatment, to remove any unsaturation and impurities from the oils, results in a product with improved thermal and thermo-oxidative stability compared to traditional solvent refined oils. See for example U.S. Pat. Nos. 5,935,417 and 5,993,644. Such products are referred to by those of skill in the art as Group II or Group III base oils. Table 1 below describes these base oil categories as set forth by the American Petroleum Institute's (hereinafter “APF”) definition for base oils.
TABLE 1Base OilViscosityCategorySulfurSaturates (%)IndexGroup I >0.03and/or <9080 to 120Group II≦0.03and ≧9080 to 90 Group III≦0.03and≧90≧120Group IVAll Polyalphaolefins(POA's)Group VAll others notincluded inGroups I, II, III,or IVSee also API Publication 1509: Engine Oil Licensing and Certification System, “Appendix E-API Base Oil Interchangeability Guidelines for Passenger Car Motor Oil and Diesel Engine Oils”.
Group II and Group III base oils, structurally different than PAO's, provide exceptional thermo-oxidative stability compared to traditional mineral base oil stocks and are more economical than PAOs. However, commonly used lubricant additives, such as amine antioxidants, phenolic antioxidants, antiwear additives, and corrosion inhibitors are less soluble in these highly saturated non-polar hydrocarbon Group II and Group III base oils. Consequently, the effectiveness of these commonly used lubricant additives is significantly reduced in Group II and Group III base oils compared to traditional mineral oils. In addition, Group II and Group III base oils lack the ability to provide swell to certain types of seals, since the refining process removes and/or destroys the naturally occurring polar compounds found in traditional solvent refined base oils that provide seal swell and compatibility. It is known in the art that these problems can be addressed by blending esters with base oils, because esters have good thermal stability as well as offer improvements both to additive solubility and seal swell characteristics. However, the addition of esters creates unacceptable hydrolytic instability in base oil/ester blends. The hydrolysis of esters to carboxylic acid in the presence of trace amounts of moisture leads to an unacceptable acceleration of base oil oxidation when used under normal conditions.
Therefore, it would be advantageous to provide a composition including Group II and/or Group III base oils which exhibits additive solvency, suitable seal swell, thermo-oxidative stability, and hydrolytic stability.
U.S. Pat. No. 5,602,086 discloses the inclusion of alkylated naphthalene blending stocks with PAO based fluids to provide desirable physical properties. It does not disclose or suggest the blending of alkylated naphthalenes with materials other than PAO, let alone that desirable physical properties could be achieved from such a blend.
Therefore, it would be unexpected that a composition including Group II and/or Group III base oils would exhibit additive solvency, suitable seal swell, thermo-oxidative stability, and hydrolytic stability.